The present invention relates to a magnetic resonance imaging apparatus (abbreviated as xe2x80x9cMRIxe2x80x9d hereinafter). In particular, it relates to an MRI apparatus for performing measurements of temperature distributions in vivo tissue.
In recent years, IVMR (Interventional MR) utilizing an MRI apparatus as a monitor has attracted attention. One of applications of IVMR is monitoring of temperature distributions in vivo tissue during laser therapy using an MRI apparatus. The techniques of imaging temperature distributions using MRI apparatuses include a method of finding temperature distributions from a signal intensity or a diffusion coefficient utilizing the fact that the signal intensity or diffusion coefficient varies with temperature change, a method of finding temperature distributions from the proton phase shift (PPS method: Proton Phase Shift method) and the like. Among these techniques, the PPS method is best in accuracy of measurement.
The PPS method utilizes the fact that when echo signals from water proton are measured, the proton phase is a function of temperature, as shown by the following equation (2). The procedure thereof is shown in FIG. 8.                               T          ⁡                      [                          xc2x0              ⁢                              xe2x80x83                            ⁢                              C                .                                      ]                          =                              φ            ⁢                          xe2x80x83                        [            degree            ]                                              TE              ⁡                              [                s                ]                                      xc3x97                          f              ⁡                              [                Hs                ]                                      xc3x97                          0.01              ⁡                              [                                                      ppm                    /                    xc2x0                                    ⁢                                      xe2x80x83                                    ⁢                                      C                    .                                                  ]                                      xc3x97                          10                              -                6                                      xc3x97                          360              ⁢                              xe2x80x83                            [              degree              ]                                                          (        2        )            
In the equation, xcfx86 (degree) represents phase, TE [s] is time between excitation of spins and generation of echo signal, f [Hz] is proton resonance frequency, and 0.01 [ppm/xc2x0C.] is the temperature coefficient of water. The same notation is used hereinafter.
An imaging sequence using, for example, the gradient echo method shown in FIG. 1 is performed first, and echo signals required for MR image reconstruction are measured (800, 810).
By performing a Fourier transformation on these echo signals, a complex image S (x, y, z) is obtained, and a phase distribution xcfx86 (x, y, z) is obtained from the arctangent of a real part and an imaginary part (801,811) of the complex image according to the equation (1).
xcfx86 (x,y,z)=tanxe2x88x921{Si(x,y,z)}xe2x80x83xe2x80x83(1)
In the equation, xcfx86 represents phase, Si is an imaginary part of a complex signal, and Sr is a real part of the complex signal. Each parameter is a function of space (x, y, z).
A temperature distribution can be found by applying the above-mentioned equation (2) to the thus obtained phase (802,812). Further, by performing subtraction of temperature distributions calculated respectively from signals obtained at different times t1 and t2, distribution of the temperature change of an examined object at a certain time can be obtained (820).
In the difference of the above-mentioned temperature distributions, difference at the background (part other than the examined object) is ideally zero since signal value of the background is zero. Actually, however, the influence of background noise cannot be eliminated from the difference. It may therefore be difficult to observe temperature change in a small area. Especially if the temperature change is to be displayed with gradation, the dynamic range becomes wider and the temperature change of a small area may be buried within the background noise.
Moreover, since the phase information is obtained by performing arctangent operation as shown in the equation (1) in the PPS method, it is theoretically possible that arctangent aliasing may be produced. When inhomogeneity of a static magnetic field is large, phase rotation increases and it becomes difficult to obtain accurate temperature distributions due to this aliasing. Furthermore, the part where the aliasing has been produced is emphasized upon performing subtraction of the above-mentioned temperature distributions, and false temperature changes, which have not actually occurred, may be observed.
An object of the present invention is to solve these problems and to provide an MRI apparatus capable of measuring temperature change distributions in an examined body precisely and which enables easy observation of the temperature change distributions.
To attain the above-mentioned objects, an MRI apparatus of the present invention comprises means for imparting a static magnetic field to an object to be examined, means for applying a plurality of gradient magnetic fields to the object, means for applying radio-frequency pulses for generating nuclear magnetic resonance in atomic nuclei of atoms constituting the object, detecting means for measuring nuclear magnetic resonance (NMR) signals from the object as complex signals, operating means for performing operations on the complex signals, and display means for displaying a result of the operations, wherein the operating means comprises means for extracting an object part from image data obtained from the complex signals, means for calculating a spatial phase distribution for the extracted part, means for calculating a temperature distribution based on the spatial phase distribution, means for calculating a temperature change distribution from the difference between data obtained in different measurements, and means for depicting the temperature change distribution.
By extracting only the object part from the image data prior to performing subtraction of data obtained in different measurements, the influence of background noise can be removed completely and information on the part with little temperature change can be reliably imaged. Means for calculating a temperature change distribution may calculate the temperature change distribution by first finding spatial phase distributions and temperature distributions from the image data obtained in two measurements respectively and performing subtraction thereof. Alternatively, it may perform subtraction of image data obtained in two measurements and then determine spatial phase difference distribution and temperature change distribution.
According to the preferred embodiment of an MRI apparatus of the present invention, means for calculating a spatial phase distribution calculates the above-mentioned spatial phase distribution according to the following equation (1),
xcfx86(x,y,z)=tanxe2x88x921{Si(x,y,z)/Sr(x,y,z)}xe2x80x83xe2x80x83(1)
(In the equation, xcfx86 represents phase, Si is an imaginary part of a complex signal, and Sr is a real part of the complex signal. Each parameter is a function of space (x, y, z).) and includes means for correcting arctangent aliasing produced in the spatial phase distribution.
By correcting the arctangent aliasing when spatial phase distribution is measured, error caused by the arctangent calculation of signals can be reliably eliminated.